If you have a mortgage on your home, you can deduct the interest from your taxes. It's a popular, well-entrenched policy. But according to one policy adviser to a U.S. senator, "the mortgage-interest deduction, from a purely policy perspective ... makes no sense."

Yesterday, we reported on the fundraisers that lobbyists hold for Congressmen every day in Washington. Today, we hear what happens inside those events. The stories are part of our series on money in politics.

Disgraced lobbyist Jack Abramoff has been making the rounds lately. He's out of prison. He has a new book. He's in a talkative mood. So I figured it was a good time to ask him about the business of lobbying — not about what he did that was illegal, but about the ordinary, legal stuff.

The firm he worked for was called Greenberg Traurig. I chose a year at random when Abramoff was working there, and picked a client I hoped would be fairly typical. I chose Tyco International, a multinational corporation that in 2003 gave Abramoff's firm $1.3 million.

Corporations don't lobby Congress for fun. They lobby because it helps their bottom line. Getting a regulation gutted or a tax loophole created means extra cash for the corporation. But getting laws changed can be very expensive. How much money does a corporation get back from investing in a good lobbyist?

It's a messy, secretive system so it was always hard to study. But in 2004, economists found a bill so simple, so lucrative, that they could finally track the return on lobbying investment.

The congressional supercommittee announced Monday that it failed to come to an agreement on reducing the deficit. After three months of negotiating, the Democrats and Republicans just couldn't agree on how much spending to cut or how high to raise taxes.

But this is not a story about how the left and right disagree with each other. In fact, they actually largely agree.